1. Field of the Invention
The present invention relates to a ladle which is used in a converter process to convey molten steel received from a converter and, more particularly, to a method of heating a ladle.
2. Description of the Related Art
A description will be given first of a conventional art.
1) Referring to FIG. 3, a ladle 1 used in a converter process is used to supply molten steel to a continuous casting process and is thereafter moved to a slag discharge station B1 by means of a crane 2 or the like. At the slag discharge station B1, the ladle 1 is tilted so that slag remaining in the ladle is discharged. The ladle is then moved to an inspection/maintenance station (not shown) where a sliding nozzle is scrubbed or replaced with a new sliding nozzle. The ladle is then moved to a pre-heating station C1 where the ladle 1 is pre-heated by means of, for example, a burner (not shown) to dehydrate the ladle 1 and make up for any reduction of the temperature of molten steel which is to be received from a converter 3.
The ladle 1 is then moved by, for example, the crane 2 mounted on a steel carrier ladle truck 5 which transports the ladle 1 to a tapping station D1. The ladle 1 which has been moved to the tapping station D1 is stationed for a predetermined period of time and, thereafter; receives molten steel directly from converter 3. After receiving the molten steel, the ladle 1 is again moved by the ladle truck 5 to a secondary refining station (not shown) where the molten steel in the ladle 1 is subjected to a secondary: refining performed by, for example, an RF method.
Subsequently, the ladle 1 on the ladle truck 5 is conveyed by, for example, the crane 2 to a continuous casting station A1. The ladle 1 conveyed to this station A1 is mounted on a continuous casting machine, and a sliding nozzle provided on the bottom of the ladle 1 is opened and closed, whereby the molten steel is continuously teemed into a tundish at an appropriate rate, so as to be cast continuously. The ladle 1 is then subjected again to the described process.
The tapping temperature at which the molten steel is discharged from the converter 3 is so determined and controlled that the molten steel is maintained high enough to enable the casting until the end of the continuous casting. As a consequence, the tapping temperature is largely ruled by the reduction in the temperature which the molten steel 1 sustains while the molten steel is held in the ladle 1.
In the conventional converter process, however, a considerably long time is involved from the pre-heating of the ladle 1 in the pre-heating station C1 until the ladle 1 receives the molten steel at the tapping station D1. In particular, the temperature of the ladle refractory is lowered due to natural heat dissipation while the ladle 1 is stationed for receiving the molten steel at the tapping station. This causes a large temperature drop of the molten steel received in the ladle 1. This requires the tapping temperature at which the molten steel is discharged from the converter to be set at a high level so that the molten steel temperature is high enough for casting even at the end of continuous casting. As a result, a greater amount of carbonaceous material such as coke, which is supplied into the molten steel to act as a temperature-raising material during blowing in the converter process, is consumed.
In addition, a greater degree of thermal attack is caused on the ladle refractory lining, due to the large difference between the temperature of the ladle refractory lining and the tapping temperature at which the molten steel is discharged from the converter, with the result that the refractory lining cannot be sustained for extended use. Further, the molten steel in the ladle 1 exhibits large local variations in temperature.
Furthermore, pre-heating the ladle at the pre-heating station requires a long time and, hence, consumes a large quantity of combustion gas (C gas) for pre-heating.
The present invention is aimed at overcoming these problems of the known art. Thus, it is an object of the present invention to provide a method of heating a ladle which permits the tapping temperature at which the molten steel is discharged from a converter to be set to a low level to permit reduction in the consumption of carbonaceous material, while suppressing thermal attack on the ladle refractory material to improve the unit ratio of the refractories, and which reduces consumption of the combustion gas used for heating the ladle by burners, thus contributing to saving energy.
2) A heating method has been known for heating a ladle by means of regenerative-type burners while closing the top opening of the ladle by means of a ladle lid on which the burners are mounted. This type of heating method is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 7-112269.
This heating method employs a pair of burner units which alternately supply fresh air and discharges combustion exhaust gas,.while recovering heat through a heat regenerator disposed therebetween. These burner units are mounted on the ladle lid which closes the top opening of the ladle. The pair of burner units alternately perform combustion. While one of the burner units is operating to heat the ladle, the combustion gas after the heating is exhausted and recovered through an exhaust gas pipe which runs through a heat regenerator which is associated with the other burner unit.
In a steady operation of this type of regenerative-type burner equipment, the rate of recovery of the exhaust gas is set to be almost equal to the rate of supply of the combustion air, for the reason stated below. Recovery of the exhaust gas at a rate in excess of the rate of supply of the combustion air causes the exhaust gas temperature at the heat-accumulator outlet to rise to an extraordinarily high level, beyond temperatures which can be sustained by structural members supporting the heat regenerator and devices arranged in the exhaust gas pipe such as a change-over valve and an exhaust fan. This makes the whole heating system inoperative and impractical. For this reason, the rate of recovery of the combustion exhaust gas is controlled to be almost equal to the rate of supply of the combustion air, from the beginning to the end of combustion.
This controlling method, however, suffers from the following disadvantage. Namely, at the beginning of combustion, most of the exhaust gas recovered through the exhaust gas pipe is used for heating the heat regenerator. In this state, the temperature of the combustion air after the heat exchange across the heat regenerator is considerably lower than the temperature of the exhaust gas collected from the ladle, so that the heat recovery ratio is undesirably low. With this controlling method, it is impossible to rapidly heat the ladle in a short time, because the combustion temperature and, hence, the combustion gas temperature cannot be raised in the beginning period of the combustion.
In view of this problem, another object of the present invention is to provide a quick heating method for rapidly heating a ladle by means of a regenerative-type burner system, wherein the high temperature of the atmosphere in the ladle is maintained without allowing the combustion gas at the heat-accumulator outlet to exceed the temperature tolerable by the heat regenerator supporting structure and the devices in the exhaust gas pipe such as a change-over valve, thus achieving high heating efficiency for heating the ladle.
3) In the known art for heating the ladle, the ladle is transported to a predetermined station by means of a truck, where the top opening of the ladle is closed by the ladle lid on which burners are mounted. Heating the ladle is conducted by combustion of a fuel by means of the burner system on the ladle lid closing the top opening of the ladle, while the combustion gas is exhausted therefrom. Movement of the ladle lid carrying the burner system is performed by means of a crane or the like.
The work for moving the ladle lid with the burner system onto and from the ladle is extremely laborious and time-consuming. In addition, there is a risk that the brim of the top opening of the ladle may be damaged by an impact produced when the ladle lid carrying the burner system is placed on the ladle.
The invention also is contemplated to overcome this problem. Thus, still another object of the present invention is to provide a ladle lid lifting apparatus for lifting and lowering a ladle lid carrying a burner system, which facilitates the work for opening and closing the top opening of a ladle with the ladle lid, while avoiding damaging of the brim of the top opening of the ladle.
1. First aspectxe2x80x94quick heating of ladle by regenerative-type burner system
To these ends, according to one aspect of the present invention, there is provided a method of heating a ladle in a process in which the ladle after teeming for continuous casting and subsequent slag discharge is mounted on a ladle truck or mover and then moved by the ladle truck to a tapping station, the ladle on the ladle truck being then stationed over a predetermined stand-by time, the ladle then being moved to a tapping position to receive a molten steel from a converter, the heating being executed before the ladle receives the molten steel from the converter. In accordance with this method, the ladle is quickly heated within the predetermined stand-by time in which the ladle is stationed in the tapping station.
Preferably, heating is performed by means of a regenerative-type burner system carried by a ladle lid which is attached to the ladle to cover the top opening of the ladle.
2. Second aspectxe2x80x94Prevention of temperature drop of ladle
In accordance with a second aspect, there is provided a method of heating a ladle in a process in which the ladle after teeming for .continuous casting and subsequent slag discharge is mounted on a ladle truck and then moved by the ladle truck to a tapping station, the ladle on the ladle truck then being stationed over a predetermined stand-by time, the ladle then being immediately moved to a tapping position to receive a molten steel from a converter, the ladle then being conveyed by the ladle truck to a secondary refining station and, after the secondary refining, moved further to the continuous casting station to teem the molten steel for the continuous casting.
The ladle heating method comprises quickly heating the ladle within a predetermined period in which the ladle is stationed at a tapping station where the ladle is to receive a molten steel from a converter, by means of a burner system mounted on a first ladle lid for covering and closing the top opening of the ladle; and keeping the top opening of the ladle covered by a second-ladle lid in operational phase other than slag discharging, quick heating, tapping and secondary refining.
3. Third aspect of the Inventionxe2x80x94Heat balance on regenerative-type burner
In accordance with a third aspect of the present invention, there is provided a method of quickly heating a ladle by means of a regenerative-type burner system, comprising the steps of: closing a top opening of the ladle by means of a ladle lid carrying the burner system, the burner system having a pair of burner units each having a heat regenerator, the burner units being alternately operable such that, when one of the burner units is activated to perform combustion, supply of the combustion air and the discharge of the combustion exhaust gas are conducted through the heat regenerator of the other burner unit; alternately activating the burner units to perform combustion while the top opening of the ladle is kept closed by the ladle lid; recovering the combustion exhaust gas through an exhaust gas pipe via the heat regenerator of the burner which is not operating; and controlling the rate of recovery of the combustion exhaust gas by controlling a flow rate control valve provided in the exhaust gas pipe, based on the temperature of the combustion exhaust gas measured at the outlet of the heat regenerator.
There is provided also a method of quickly heating a ladle by means of a regenerative-type burner system, comprising the steps of: closing a top opening of the ladle by means of a ladle lid carrying the burner system, the burner system having a pair of burner units each having a heat regenerator, the burner units being alternately operable such that, when one of the burner units is activated to perform combustion, supply of the combustion air and the discharge of the combustion exhaust gas are conducted through the heat regenerator of the other burner unit; alternately activating the burner units to perform combustion while the top opening of the ladle is kept closed by the ladle lid, while recovering the combustion exhaust gas through an exhaust gas pipe via the heat regenerator of the burner which is not operating; and controlling a flow rate control valve provided in the exhaust gas pipe, in accordance with a flow rate pattern of the combustion exhaust gas flowing through the exhaust gas pipe, the flow rate pattern being set up beforehand based on the relationship between the temperature of the combustion exhaust gas at the outlet of the heat regenerator and the rate of recovery of the combustion exhaust gas.
The regenerative-type burner units may be provided with pilot burners. Before the regenerative-type burners are activated, the pilot burners are operated to perform combustion, thereby pre-heating the regenerators.
4. Fourth Aspect of the Inventionxe2x80x94Control of tapping temperature
In accordance with a fourth aspect of the present invention, there is provided a method of heating a ladle in a process in which a ladle after teeming for continuous casting and subsequent slag discharge is mounted on a ladle truck and then moved by the ladle truck to a tapping station, the ladle on the ladle truck being then stationed over a predetermined stand-by time, the ladle being then immediately moved to a tapping position to receive molten steel from a converter, the heating of the ladle being performed before the ladle receives the molten steel from the converter, the heating method comprising the steps of: quickly heating, during the predetermined stand-by time, the ladle with regenerative-type burner system carried by a ladle lid attached to the ladle to cover the top opening of the ladle; determining the amount of heat possessed by the ladle refractory material based on the amount of heat input and the sensible heat carried by the exhaust gas; determining, based on the amount of heat possessed by the ladle refractory material, the tapping rate at which the molten steel is discharged from the converter and the specific heat of the molten steel, a molten steel cool-down prevention temperature given to the ladle by the quick heating of the ladle; and controlling the tapping temperature at which the molten steel is discharged from the converter, in accordance with the molten steel cool-down prevention temperature.
5. Fifth Aspect of the Inventionxe2x80x94Ladle lid lifting apparatus
In accordance with a fifth aspect of the present invention, there is provided a ladle lid lifting apparatus for lifting and lowering a ladle lid to open and close a top opening of a ladle that has been moved to and stationed at a predetermined position by a ladle truck, the ladle lid being provided with a burner system, the ladle lid lifting apparatus comprising: a supporting frame arranged to straddle over the path of the ladle truck carrying the ladle stationed at the predetermined position; a first chain or suspender supporting and suspending the ladle lid with the burner system for substantially vertical movement, the first chain extending upward from the ladle lid and then substantially horizontally after turning a first sprocket carried by the supporting frame, the end portion of the substantially horizontal extension of the first chain being connected to a connecting member; a second chain or suspender connected to the connecting member and extending substantially horizontally away from the first chain and then downward after turning a second sprocket carried by the supporting frame, the end portion of the downward extension of the second chain being connected to a counter weight having a weight which substantially balances the weight of the ladle lid inclusive of the burner system; driving means for driving the second sprocket to cause substantially vertical movement of the ladle lid with the burner system; guiding means for guiding the ladle lid with the burner system when the ladle lid moves up and down; and a combustion air supply pipe, an exhaust gas pipe and a fuel gas supply pipe connected to the burner system on the ladle lid, the combustion air supply pipe, exhaust gas pipe and the fuel gas supply pipe having substantially vertically extending portions including bellows that accommodate the vertical movement of the ladle lid.